As a proton-conductive complex oxide for use in a fuel cell, Li1.7H5.05La3Nb0.25Zr1.75O12 has been known (Non-Patent Document 1). Li1.7H5.05La3Nb0.25Zr1.75O12 is prepared by washing Li6.75La3Nb0.25Zr1.75O12 having good lithium conductivity with water, subjecting the complex oxide to vacuum drying, and repeatedly subjecting the complex oxide to an annealing treatment at 150° C.
However, regarding the crystal structure of Li1.7H5.05La3Nb0.25Zr1.75O12, only the lattice constant has been reported and a correlation between a hydrogen amount and proton conductivity has not been clarified. In addition, Non-Patent Document 2 discloses that lithium can be substituted by hydrogen by washing Li6.75La3Ta0.25Zr1.75O12 with water at room temperature. However, the amount of substitution to hydrogen is as small as 21% of the amount of lithium.